1. See one.
2. Do one.
3. Teach one.
Step 3 is the most importaint one as you encounter questions you would never have thought of yourself.

To teach is to learn twice. - Joseph Joubert, Pensées, 1842
Who dares to teach must never cease to learn. - John Cotton Dana
Whoever ceases to be a student has never been a student. - George Iles

ak

 

My project I am creating a system that should be battery powered, at most 30-50 VAC.
I am generating square waves to stimulate human arm.
I need current to be constant between 0 - 10 mA across human skin resistance.
Therefore I need a constant current source.
I also need current to be going both direction.

I think constant current source could be either op amp based or transistor based?
What is difference?
Do they all use varying voltage to regulate current?
Do they all allow current to go both direction?

I am new to electric engineering so I apologize if anything is unclear.
Could anyone point me a direction?
Thank you.

Also I tried

With V+ = 5V, Vcc = +-10V, using 180 ohm instead of 250 ohm, AD822 op amp.
circuit works when Rload = 0, 28 mA.
but when I put Rload as 680 ohm, current exceeds 28 mA as Vcc increase.
Anyone know what might be the error?

Search on medical electronics and Lambda diode.